This is a tough question to ask because not only does it vary dramatically by title, but it also varies by genre too. I also don't have these figures, so I asked a few pals of mine in the industry for help, and here's what they shared:

First off, Renee Wilmeth (who was the acquisitions editor on my Google book and is now helping run the cool new Literary Architects) shared this:

My understanding was always "as little as possible." That is -- I don't know that there's one set percentage that a publisher expects to write down on unsold inventory as a percentage of overall books printed. (I'm sure some business manager has a number.)

Is this a number an author is seeing on a contract? If so, I'd guess it was an average based on how many are typically remaindered for that imprint or company.

Certainly, publishers have a set number of returns they budget for. For many trade books, it's as high as 50-55%. For most midlist trade imprints, 35% is more standard. For even more specific imprints (like tech books) they're able to get those numbers down even further (maybe in the 20-25% range.) But due to the costs of holding inventory, most groups are managing reprint and inventory run rates very closely. At Penguin, we would typically look at weekly run rates and gauge reprints based on a 3-6 month supply depending on how old the title was and how quickly it was running.

Obviously, there is a unit cost improvement for a higher reprint. However, this cost savings isn't as high as one would expect based on the cost of warehousing the additional units longer. (This push-pull is so strong in some organizations that sales is tasked with keeping inventories down while editorial is tasked with keeping unit costs low setting up one of those awful push-pull intergroup wrestling matches that some management teams still seem to think works better for the company.)

As far as inventory dealt with when a book is taken out of print, it goes to the remaindering department -- and depending on the contract is often offered to the author for purchase first at the price set by the remaindering group. I talked to to a lot of authors who seemed to have this misconception that publishers purposefully printed more books than they'd need so they could remainder more copies cheating the author out of their royalties. If they could only sit in on one of those horrible wrangling sessions called "a reprint meeting", they'd know differently.

Otherwise, I can't name a percentage off the top of my head that I've ever budgeted as a percentage of net sales (or gross sales for that matter) to account for obsolete inventory -- remaindered or destroyed copies. If I had to guess, I'd say it was in the 5% range for net, or the 5-10% range for gross depending on the type of book.

Matt Wagner, head of author agency Fresh Books, added this:

I wouldn't know the nuts and bolts of reprint decisions on the publisher side like Renee and Jim, but agree that returns do still hover around 20-25% for tech titles and can run as high as 40 or 50% for some mass market titles. Looks like everyone is working hard to have better inventory control but sometimes you never really know what you're going to see coming back until the book goes out of print.

And, finally, Renee responded with a smart wrap up on this topic:

Most authors will be frustrated if they think this number is something that they have control over. It's part of a cost of doing business in publishing, and based on the margins most publishers run on, it's something that's as tightly controlled as possible since it's a cost the publisher bears, not the author. Even in our model at LitArch, we cover manufacturing, warehousing and distribution and don't have a percentage or charge that back to the author.

Thanks to both of you for your helpful insight on this topic!